Light emitting device

ABSTRACT

A light emitting device capable of improving both color unevenness and emission output power is provided. The light emitting device includes a semiconductor light emitting element including a semiconductor layer that emits primary light; and a fluorescent material layer disposed on the light emitting side of the semiconductor light emitting element, that absorbs a part of the primary light and emits secondary light having a wavelength longer than that of the primary light; and emits light of blended color of the primary light and the secondary light of the light emitting element, and further includes a scattering layer in which particles having a mean particle size D that satisfies the inequality: 20 nm&lt;D≦0.4×λ/π are dispersed in a transparent medium.

This application is a Divisional of co-pending application Ser. No.12/970,355 filed on Dec. 16, 2010, and for which priority is claimedunder 35 U.S.C. §120; which claims priority from P 2009-285971 filed inJapan on Dec. 17, 2009; the entire contents of all these applicationsbeing hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light emitting device capable ofcolor-mixing light emitted by a light emitting element and lightobtained through wavelength conversion of a part of the original light,thereby emitting light of a different color.

2. Description of the Related Art

A semiconductor light emitting element such as light emitting diode issmall in size, has high power efficiency and emits light with clearcolor. The semiconductor light emitting element also has such advantagesas excellent startup performance and high durability to vibration andrepetitive operations of turning on and off. Light emitting devices havebeen developed that are capable of emitting light of various colors bycombining the primary light of the semiconductor light emitting elementand a fluorescent material capable of emitting secondary light of adifferent wavelength through excitation by the primary light, based onthe principle of color mixing of light. Such light emitting devices areused as various light sources. Particularly in recent years, as suchlight emitting devices are spotlighted as the next-generationillumination of lower power consumption and longer service life thatreplaces the fluorescent lamps, it is required to further improve theoutput power of light emission and the emission efficiency. There isalso a demand for light source of higher brightness in projectors suchas headlight of automobile and flood lighting.

Such a light emitting device includes a semiconductor light emittingelement die-bonded onto a metallic lead frame or a ceramic substrate anda fluorescent material layer formed around the semiconductor lightemitting element by various methods such as potting, screen printing orthe like. Japanese Patent Publication No. 3,503,139, for example,discloses a light emitting device that emits white light and has highdurability and high color rendering performance, constituted bycombining a light emitting diode formed from a gallium nitride-basedcompound semiconductor that is capable of emitting blue light as theprimary light and a garnet fluorescent material activated with ceriumthat is capable of emitting yellow light as the secondary light. FIG. 10is a sectional view of a light emitting device 100 disclosed in JapanesePatent Publication No. 3,503,139, that comprises a light emitting diode102 fastened at a distal end of 105 of a pair of lead frames 105, 106,and a fluorescent material layer 101 that is formed from a resincontaining fluorescent material particles and coats the light emittingdiode. The light emitting diode 102 and the fluorescent material layer101 are coated by a bullet-shaped transparent resin 104. JapaneseUnexamined Patent Publication (Kokai) No. 2002-141559 discloses such aconstitution as a powder of silica, alumina or titania is dispersed asscattering particles in the transparent resin that coats thesemiconductor light emitting element and the fluorescent material layer,in order to mitigate color unevenness of the light emitting device thathas a semiconductor light emitting element and a fluorescent materiallayer. Japanese Unexamined Patent Publication (Kokai) No. 2009-24117discloses such a constitution as fine particles measuring 20 nm or lessthat is formed from an inorganic material having a high refractive indexare dispersed in the transparent resin and in the fluorescent materiallayer, in order to increase the refractive indices of the transparentresin and the fluorescent material layer thereby to improve lightextracting efficiency. According to Japanese Unexamined PatentPublication (Kokai) No. 2009-24117, the particle size of the fineparticles is set to 20 nm or less in order to suppress the scattering oflight. Japanese Unexamined Patent Publication (Kokai) No. 2008-130279discloses such a constitution as the particle size of fluorescentmaterial particles contained in the fluorescent material layer is set toa small size that corresponds to the Rayleigh scattering regime, therebyto improve the light extracting efficiency.

However, it has been difficult to improve the color unevenness and lightextracting efficiency of the light emitting device at the same time, inthe light emitting device of the prior art described above. First, insuch a light emitting device of the prior art as that of Japanese PatentPublication No. 3,503,139, the semiconductor light emitting element andthe fluorescent material layer are coated with the transparent resin,and epoxy, silicone or the like is often used as the transparent resin.These transparent resins show excellent optical properties such ascolorless transparency, homogeneity and high isotropy after curing,although the excellent optical properties allow the emission intensitydistribution of the semiconductor light emitting element and thefluorescent material layer to be shown faithfully to the outside.Meanwhile, a difference between the emission intensity distribution ofthe semiconductor light emitting element and the emission intensitydistribution of the fluorescent material layer is likely to arise, andit is not easy to make both emission intensity distributions to beidentical. Since chromaticity of light emitted by the light emittingdevice is determined by the ratio of intensity of the secondary lightemitted by the fluorescent material layer to the intensity of primarylight emitted by the semiconductor light emitting element, presence ofdifference in emission intensity distribution between the semiconductorlight emitting element and the fluorescent material layer causes thechromaticity to change with the position within the light emittingdevice and with the direction of viewing the light emitting device, thusresulting in color unevenness.

Japanese Unexamined Patent Publication (Kokai) No. 2002-141559 disclosesthe constitution where scattering particles are dispersed in thetransparent resin that coats the semiconductor light emitting elementand the fluorescent material layer, in order to improve the colorunevenness. When the scattering particles are dispersed in thetransparent resin, light of the semiconductor light emitting element andlight of the fluorescent material layer are scattered so that theemission intensity distribution of each member is made more uniform andthe color unevenness is suppressed. However, although a large quantityof scattering particles dispersed in the transparent resin can improvethe color unevenness, it leads to a problem of lower light extractingefficiency. That is, the scattering particles dispersed in thetransparent resin scatter the light of the semiconductor light emittingelement and the fluorescent material layer in every direction. As aresult, the proportion of light returning to the semiconductor lightemitting element and the fluorescent material layer also increases.Light returning to the semiconductor light emitting element and thefluorescent material layer are finally extracted to the outside of thelight emitting device after being reflected on various interfaces, whilenot a small proportion of the light is absorbed in this process. As aresult, intensity of light extracted to the outside of the lightemitting device at the end decreases, thus resulting in lower lightextracting efficiency. In Japanese Unexamined Patent Publication (Kokai)No. 2009-24117 and Japanese Unexamined Patent Publication (Kokai) No.2008-130279, although the means for improving light extractingefficiency is studied, no consideration is given to the color unevennesscaused by the difference between the emission intensity distribution ofthe semiconductor light emitting element and the emission intensitydistribution of the fluorescent material layer.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a novellight emitting device that can improve both the color unevenness andlight extracting efficiency of the light emitting device at the sametime.

In order to achieve the object described above, the present inventorshave intensively studied. As a result, they noticed a fundamentaldifference in emission intensity distribution between the semiconductorlight emitting element and the fluorescent material layer, and foundthat the above object can be achieved by taking advantage of thedifference.

First, the difference in emission intensity distribution betweensemiconductor light emitting element and fluorescent material layer willbe described. The emission intensity distribution involves adistribution caused by the position within the light emitting surface(hereafter referred to as in-surface distribution) and a distributioncaused by the direction of viewing the light emitting surface (hereafterreferred to as light distribution characteristic). The primary lightemitted from the semiconductor light emitting element such as lightemitting diode is surface emission from the semiconductor layer, whichis subjected to the influence of light blocking by an electrode, so thatthe in-surface distribution tends to be generated depending on the dieshape and the electrode shape, while the light distributioncharacteristic tends to be non-uniform. Also because the primary lightof the semiconductor light emitting element emits after passing throughthe fluorescent material layer, the in-surface distribution and thelight distribution characteristic of light emission of the semiconductorlight emitting element are affected also by means of the arrangement,thickness, shape or the like of the fluorescent material layer. On theother hand, the fluorescent material layer emits the secondary light ofa longer wavelength by absorbing a part of the primary light of thelight emitting diode, and therefore there is no need for complicatedstructure of electrode or the like, thus providing a high degree offreedom in the design of arrangement, thickness and shape. As a result,the in-surface distribution of the fluorescent material layer can berelatively freely controlled and the light distribution characteristicof the fluorescent material layer can be easily made uniform byadjusting the arrangement, thickness and shape or the like of thefluorescent material layer.

The difference described above becomes more conspicuous when thefluorescent material exists in the form of particles in the fluorescentmaterial layer. When the fluorescent material exists in the form ofparticles, the secondary light is emitted in every direction from thefluorescent material particles. As a result, light distributioncharacteristic exhibited by the fluorescent material tends to becomebasically uniform, and does not change with the quantity of fluorescentmaterial. Light emission distribution exhibited by the semiconductorlight emitting element is surface emission from the semiconductor layer,and therefore the emission intensity distribution thereof tends to benon-uniform, and changes significantly with the quantity of fluorescentmaterial disposed around the semiconductor light emitting element. Thisis because the fluorescent material particles behave as a scatteringobject as well as absorber of light of the semiconductor light emittingelement. Therefore, when the quantity of fluorescent material is changedso as to change the ratio of light intensity of the semiconductor lightemitting element and the light intensity of the fluorescent materiallayer and obtain the desired chromaticity, emission intensitydistribution of the semiconductor light emitting element changessignificantly as a consequence.

The present invention has been made by noticing the fundamentaldifference in the emission intensity distribution exhibited by thesemiconductor light emitting element and the fluorescent material layer,and provides a light emitting device comprising a semiconductor lightemitting element including a semiconductor layer that emits primarylight; and

a fluorescent material layer disposed on the light emitting side of thesemiconductor light emitting element, that absorbs a part of the primarylight and emits secondary light having a wavelength longer than that ofthe primary light, the light emitting device emitting light of blendedcolor of the primary light and the secondary light, wherein

the light emitting device further comprises a scattering layer, in whichparticles are dispersed in a transparent medium, on the light emittingside of the fluorescent material layer, said particles having a meanparticle size D that satisfies [Inequality 1]:

20 nm<D≦0.4×λ/π  [Inequality 1]

λ is the wavelength of the primary light propagating in the transparentmedium) and

the scattering layer scatters the primary light and causes it to emitfrom the light emitting device.

The present invention is based on the notice of the fact that emissionintensity distribution of the fluorescent material layer can easily becontrolled by means of the arrangement, thickness and shape or the likeof the fluorescent material layer, while emission intensity distributionof the semiconductor light emitting element tends to be non-uniformfundamentally and is affected also as the light passes throughfluorescent material layer. Accordingly the present invention ischaracterized by the capability to selectively scatter the primary lightof the semiconductor light emitting element by controlling the meanparticle size D of the particles, dispersed in the transparent medium,within a predetermined range. To selectively scatter the primary lightof the semiconductor light emitting element means scattering the primarylight of the semiconductor light emitting element significantly morestrongly than the secondary light of longer wavelength of thefluorescent material layer. It is preferable that the primary light isscattered twice more strongly than the secondary light.

Selectively scattering the primary light of the semiconductor lightemitting element makes it possible to selectively control the emissionintensity distribution of the semiconductor light emitting element.Therefore, since the present invention enables it to appropriatelycontrol the emission intensity distribution of the fluorescent materiallayer by means of the arrangement, thickness and shape thereof, andcontrol the emission intensity distribution of the semiconductor lightemitting element by means of the scattering layer independently of theemission intensity distribution of the fluorescent material layer, it ismade possible to suppress the color unevenness by reducing thedifference in emission intensity distribution between the semiconductorlight emitting element and the fluorescent material layer. Alsoaccording to the present invention, since light of the fluorescentmaterial layer is not scattered strongly by the scattering layer, it ismade possible to reduce the light that returns to the semiconductorlight emitting element and the fluorescent material layer due toexcessive scattering, and suppress the light extracting efficiency fromdecreasing. As a result, the present invention makes it possible tosuppress light extracting efficiency of the light emitting device fromdecreasing, thereby to improve the color unevenness and provide thelight emitting device that is bright and has less color unevenness.

The particles used in the present invention also have an effect ofincreasing the refractive index and the heat conductivity of thematerial in which the particles are dispersed, while suppressingexcessive scattering of visible light therein. Accordingly, reflectionloss may also be reduced by dispersing the particles in variousmaterials other than the scattering layer. For example, the particlesmay be dispersed in the fluorescent material layer per se, an adhesivethat fastens the fluorescent material layer onto the semiconductor lightemitting element and the die-bonding material that fastens thesemiconductor light emitting element. The particles dispersed in thesematerials may have the same as or different from that of the particlesdispersed in the scattering layer, as long as the mean particle sizesatisfies Inequality 1.

As described above, according to the present invention, in the lightemitting device including the semiconductor light emitting element andthe fluorescent material layer, since the scattering layer that scattersthe primary light of the semiconductor light emitting element morestrongly than the secondary light of the fluorescent material layer isprovided, it is made possible to reduce the return light caused byexcessive scattering of the secondary light of the fluorescent materiallayer, and suppress the light extracting efficiency from decreasing,while suppressing the color unevenness caused by the difference inemission intensity distribution between the semiconductor light emittingelement and the fluorescent material layer. As a result, the lightemitting device that emits bright light with less color unevenness canbe achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing a light emitting deviceaccording to the first embodiment of the present invention.

FIG. 2 is a schematic sectional view showing an example of a lightemitting element used in the light emitting device of FIG. 1.

FIG. 3 is a graph showing a relation between the particle size D ofparticles and the scattering coefficient ks.

FIG. 4 is a schematic sectional view showing a light emitting deviceaccording to the second embodiment of the present invention.

FIG. 5 is a schematic sectional view showing a light emitting deviceaccording to the third embodiment of the present invention.

FIG. 6 is a schematic sectional view showing a light emitting deviceaccording to the fourth embodiment of the present invention.

FIG. 7 is a schematic sectional view showing a light emitting deviceaccording to the fifth embodiment of the present invention.

FIG. 8 is a schematic sectional view showing a light emitting deviceaccording to the sixth embodiment of the present invention.

FIG. 9 is a schematic sectional view showing a light emitting deviceaccording to the seventh embodiment of the present invention.

FIG. 10 is a schematic sectional view showing a light emitting device ofthe prior art.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the present invention will be described belowwith reference to the accompanying drawings. The drawings show theobjects schematically, and information included therein such as layout,dimensions, proportion and shape may be different from the actual. Themembers using the same reference numeral as that of another embodimentin the respective embodiments denote the same or corresponding members,and description thereof may be omitted.

In the present specification, the term “refractive index” means thevalue of refractive index observed for the wavelength of the primarylight emitted by the semiconductor light emitting element. Thedescription that a material is transparent means that the material hassufficiently transparency to the primary light of the semiconductorlight emitting element and the secondary light of the fluorescentmaterial layer, and the mixed light of the primary light of thesemiconductor light emitting element and the secondary light of thefluorescent material layer can pass therethrough and function as thelight source. “Mixing” of light means mixing of light having differentvalues of chromaticity in space so that the resultant light is perceivedby the human eye as light having chromaticity different from that ofeach original light.

In the present specification, terms “up” and “down” are used also toindicate the side of the light emitting device where emitted light isextracted and the opposite side, respectively. For example, “upward”indicates the direction of the light emitting device where emitted lightis extracted, and “downward” indicates the opposite direction. Also,“Top surface” indicates the surface one the side of the light emittingdevice where light is extracted, and “bottom surface” indicates thesurface on the opposite side. Terms “side face” and “sideways” indicatethe surface or direction perpendicular to the above “top surface” andthe “bottom surface.” Term “inside” used in relation to the lightemitting device means a position nearer to the light emitting layer ofthe light emitting device and “outside” means a position on the oppositeside.

First Embodiment

FIG. 1 is a schematic sectional view showing a light emitting device 1according to the first embodiment of the present invention. Asemiconductor light emitting element 2 is fastened via a solder bump 8on a mounting substrate 10, and the periphery is coated by a fluorescentmaterial layer 16 having substantially uniform thickness. Thesemiconductor light emitting element 2 is constituted by forming asemiconductor layer 6, that has a light emitting layer capable ofemitting blue light, on a substrate 4 of square shape in plan view, andis flip-chip mounted with the semiconductor layer 6 facing down and thesubstrate 4 facing up. The fluorescent material layer 16 is formed bydispersing fluorescent material particles 14 in a transparentfluorescent material holding member 12, so that the fluorescent materialparticles 14 absorb a part of blue light (primary light) emitted by thesemiconductor light emitting element 2 and emit light of longerwavelength such as yellow light (secondary light). Formed on the lightemitting side of the fluorescent material layer 16 is a scattering layer21 that coats the semiconductor light emitting element 2 and thefluorescent material layer 16 as a whole. FIG. 2 is a schematicsectional view showing the structure of the semiconductor light emittingelement 2, constituted by sequentially laminating an n-sidesemiconductor layer 22, a light emitting layer 24 and a p-sidesemiconductor layer 26 successively on the substrate 4 such as sapphirethat has transparent and insulating properties. The p-side semiconductorlayer 26 and the active layer 24 are partially removed to expose then-side semiconductor layer 22, and an n-side electrode 32 is formed onthe exposed surface. A reflective electrode 28 is formed oversubstantially the entire surface of the p-side semiconductor layer 26,and also a p-side electrode 30 for connection to the outside is formed.Film 34 is an insulating protective film.

With the light emitting device 1 having such a constitution, emission oflight having desirable chromaticity can be obtained by color mixing theprimary light emitted by the semiconductor light emitting element 2 andthe secondary light having a longer wavelength emitted by thefluorescent material layer 16. When the fluorescent material layer 16emits yellow light, for example, it is blended with the blue lightemitted by the semiconductor light emitting element 2 thereby to obtainwhite light. In the light emitting device 1 of the present embodiment,since the top surface and four side faces of the semiconductor lightemitting element 2 mounted on the mounting substrate 10 are coated bythe transparent material without blocking the light, surfaces of thelight emitting device that oppose the top surface and four side faces ofthe semiconductor light emitting element 2 all serve as light extractingsurfaces, thus enabling it to extract light with high efficiency.Particularly in the present embodiment, the semiconductor light emittingelement 2 is coated by the scattering layer 21 of semi-spherical domeshape. As a result, when the semiconductor light emitting element 2 isregarded as a point light source, light emits perpendicularly to thesurface of the scattering layer 21 in every direction, so thatreflection loss at the surface of the scattering layer 21 is reduced andlight extracting efficiency increases. However, when there is adifference between the emission intensity distribution exhibited by thesemiconductor light emitting element 2 and the emission intensitydistribution exhibited by the fluorescent material layer 16, colorunevenness occurs as the chromaticity changes with the position withinthe light emitting device 1 and with the direction of viewing the lightemitting device 1. When light is extracted over a range of 180 degreesaround the optical axis of the semiconductor light emitting element 2 asshown in FIG. 1, in particular, color unevenness due to the differencein light distribution characteristic between the semiconductor lightemitting element 2 and the fluorescent material layer 2 becomesconspicuous since light is emitted not only through the top of thesemiconductor light emitting element 2 but also through the side facesthereof.

As shown in FIG. 1, the fluorescent material layer 16 is formed to coatthe top surface and four side faces of the semiconductor light emittingelement 2 with substantially uniform thickness, thereby to emit thesecondary light of longer wavelength in every direction from thefluorescent material particles 14 provided in the fluorescent materiallayer 16. Therefore, difference in intensity is relatively small betweenthe secondary light emitted upward from the fluorescent material layer16 and the secondary light emitted sideways. Also the fluorescentmaterial layer 16 is uniform regardless of the position therein becauseits structure does not change with the position in the surface, andtherefore shows an emission intensity distribution that is relativelyuniform. In contrast, the semiconductor light emitting element 2 emitsthe primary light by surface emission from the thin light emitting layer24 included in the semiconductor layer 6. As a result, the lightdistribution characteristic of the primary light has such acharacteristic distribution that is high upward and horizontaldirections and is low in oblique directions. Since the semiconductorlight emitting element 2 also has a complicated electrode structure asshown in FIG. 2, a current density distribution tends to generate in thelight emitting layer 24 and an influence is exerted by the blocking andabsorption of light by the electrodes 30, 32. Accordingly, the primarylight emitted by the semiconductor light emitting element 2 has strongin-surface distribution of emission. As a result, there is differencebetween the emission intensity distribution of the fluorescent materiallayer 16 and the emission intensity distribution of the semiconductorlight emitting element 2, thus resulting in color unevenness.

In the present embodiment, light returning from the fluorescent materiallayer 16 can be reduced to achieve light emission of high efficiency, byslightly coating the periphery of the semiconductor light emittingelement 2 with the fluorescent material layer 16 having substantiallyuniform thickness. As a result, the difference between the emissionintensity distribution of the fluorescent material layer 16 and theemission intensity distribution of the semiconductor light emittingelement 2 tends to increase, thus aggravating the problem of colorunevenness. That is, when the periphery the semiconductor light emittingelement 2 is coated with the fluorescent material layer 16 havingsubstantially uniform thickness, the fluorescent material layer 16 hasone flat light emitting surface substantially parallel to the principalsurface of the semiconductor light emitting element 2 on the lightemitting side thereof, and four flat light emitting surfacessubstantially parallel to the side faces of the semiconductor lightemitting element 2. Since the fluorescent material layer 16 has uniformintralayer structure as described above, intensity of the secondarylight emitted by the fluorescent material layer 16 depends on the lengthof the optical path of the primary light passing through the fluorescentmaterial layer 16. When the fluorescent material layer 16 has the flatlight emitting surfaces nearly parallel to the principal surface of thesemiconductor light emitting element 2 on the light emitting sidethereof, and the flat light emitting surfaces nearly parallel to theside faces of the semiconductor light emitting element 2 as in thepresent embodiment, although the optical path length passed through thefluorescent material layer 16 by the primary light emitting at rightangles from each surface of the semiconductor light emitting element 2agrees with the thickness of the fluorescent material layer 16, theoptical path length passed through the fluorescent material layer 16 bythe primary light emitting obliquely from each surface of thesemiconductor light emitting element 2 is longer than the thickness ofthe fluorescent material layer 16. As a result, emission intensitydistribution of the secondary light emitted by the fluorescent materiallayer 16 shows higher intensity in the direction oblique to surface ofthe semiconductor light emitting element 2 than in the directionperpendicular to the surface of the semiconductor light emitting element2. However, the situation is commonly opposite for the emissionintensity distribution of the primary light emitted by the semiconductorlight emitting element 2. Namely, emission intensity is lower in thedirection oblique to each surface of the semiconductor light emittingelement 2 than in the direction perpendicular to each surface of thesemiconductor light emitting element 2. As a result, the differencebetween the emission intensity distribution of the fluorescent materiallayer 16 and the emission intensity distribution of the semiconductorlight emitting element 2 becomes more conspicuous and color unevennesstends to be more significant.

Accordingly, the light emitting device 1 of the present embodiment ischaracterized in that the primary light emitted by the semiconductorlight emitting element 2 is caused to emit to the outside after beingselectively scattered by the scattering layer 21 formed around thesemiconductor light emitting element 2. Selective scattering of theprimary light of the semiconductor light emitting element 2 makes theemission intensity distribution of the semiconductor light emittingelement more uniform. As a result, the difference in the emissionintensity distribution between the semiconductor light emitting element2 and the fluorescent material layer 16 can be decreased so as tosuppress color unevenness. Also because the secondary light emitted bythe fluorescent material layer 16 is not strongly scattered by thescattering layer 21, the light that returns to the semiconductor lightemitting element 2 and the fluorescent material layer 16 due toexcessive scattering of the secondary light can be decreased, and lightextracting efficiency can also be suppressed from decreasing. As aresult, the light emitting device that emits bright light with lesscolor unevenness by simultaneously improving the color unevenness andthe light extracting efficiency can be achieved.

The effect of improving the color unevenness becomes more remarkablewhen the angle of light emission of the light emitting device 1 aroundthe semiconductor light emitting element 2 increases, as in the presentembodiment. The light emission angle θ of the light emitting device 1refers to the angular range, with the semiconductor light emittingelement located at the vertex of the angle, in which light emits fromthe light emitting device, and θ becomes 2α when light emits over anangular range defined by angle α from the optical axis of the lightemitting device 1. In the case of the present embodiment, θ=180 degrees.Since the light emitting layer 3 8 of the semiconductor light emittingelement 2 is a thin layer, as described above, the light distributioncharacteristic of light emitted from the semiconductor light emittingdevice 2 has such a characteristic distribution that is high in upwardand horizontal directions and is low in oblique directions. Therefore,when the light emitting device has a wide light emission angle, colorunevenness due to the light distribution characteristic of thesemiconductor light emitting element 2 is likely to appear. On the otherhand, if color unevenness can be suppressed, it is more advantageous fora light source, in many cases, that the light emitting device has awider angular range of emission. In the present embodiment, since thelight distribution characteristic of the semiconductor light emittingelement 2 can be improved by means of the scattering layer 21, colorunevenness can be suppressed even when the light emission angle of thelight emitting device 1 is increased. According to the presentinvention, the light emission angle of the light emitting device 1, withthe semiconductor light emitting element 2 located at the vertex of theangle, is 120° or more, preferably 150° or more, and most preferably180° or more.

The scattering layer 21 is constituted by dispersing the particles 20 inthe transparent medium 18 made of a transparent resin or glass. Bydispersing the particles 20 having a mean particle size D that satisfiesInequality 1, it is made possible to selectively scatter only theprimary light emitted by the semiconductor light emitting element 2. InInequality 1 shown below, is a wavelength that the primary light emittedby the semiconductor light emitting element 2 shows when it propagatesin the transparent medium 18.

20 nm<D≦0.4×λ/π  [Inequality 1]

Regarding scattering theory, the particles 20 scatter light in differentmodes depending on the value of size parameter α=πD/λ: Rayleighscattering when α≦0.4, Mie scattering when 0.4<α<3 and diffractivescattering when α is 3 or more. Therefore, the primary light emitted bythe semiconductor light emitting element 2 undergoes Rayleigh scatteringwhen a particle size D of the particles 20 is (0.4×λ/π) or less. Forexample, when the refractive index of the transparent medium 18 is from1.4 to 1.5, blue light having a wavelength of 450 nm in air undergoesRayleigh scattering regime when the particle size D is about 40 nm orless.

In the Rayleigh scattering regime, scattering coefficient ks is given bythe following equation.

$\begin{matrix}{k_{s} = {\frac{2\pi^{6}}{3}{n\left( \frac{m^{2} - 1}{m^{2} + 2} \right)}^{2}\frac{D^{5}}{\lambda^{4}}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

n in the above formula is the number of particles 20 per 1 cm³, m is areflection coefficient, D is a particle size of the particle 20 and λ isa wavelength of light in the medium 18 in which the particles 20 aredispersed. As shown by this formula, since scattering intensity in theRayleigh scattering regime is inversely proportional to the fourth powerof the wavelength λ of the light, light of smaller wavelength isscattered more strongly. Therefore, the primary light having shorterwavelength emitted by the semiconductor light emitting element 2 isscattered more strongly than the secondary light of longer wavelengthemitted by the fluorescent material layer 16.

FIG. 3 is a graph showing a relation between the particle size D and thescattering coefficient ks for light having a wavelength of 450 nm and550 nm when the number n of the particles 20 is 8,500,000,000, therefractive index of the particles 20 is 2.15 and the refractive index ofthe transparent medium 18 is 1.41. As shown in FIG. 2, light having awavelength of 450 nm is scattered twice or more strongly as light havinga wavelength of 550 nm, when the particle size D of the particles is 40nm or less, namely falls within the Rayleigh scattering regime. However,the scattering coefficient ks decreases as the particle size D becomessmaller and, when the particle size D of the particles 20 is 20 nm orless, intensity of scattering decreases for the light of eitherwavelength. Accordingly in the present embodiment, the mean particlesize D of the particles 20 is set 20 nm or more. When the mean particlesize of the particles is 20 nm or more, a desired level of scatteringintensity for the primary light in the visible range can be obtained byappropriately controlling the number of particles. When the mean size ofthe particles is 20 nm or less, a desired level of scattering intensitycannot be obtained unless a significantly larger number of particles areprovided, while too large number of particles make it difficult todisperse them in the resin. Thus the primary light of the semiconductorlight emitting element 2 can be scattered selectively over the secondarylight of longer wavelength emitted by the fluorescent material layer 16,when the particle size D of the particles 20 is set more than 20 nm and(0.4×λ/π) nm or less.

In the light emitting device 1 shown in FIG. 1, an inorganic materialhaving a higher refractive index than that of the transparent medium 18is used as the particles 20 in the scattering layer 21. For example, asilicone resin or epoxy resin having a refractive index in a range from1.41 to 1.53 may be used for the transparent medium 18, and zirconiahaving a refractive index of 2.15 may be used for the particles. Thisincreases the effective refractive index of the scattering layer 21 andimproves the light extracting efficiency of the light emitting device 1.In the light emitting device 1 shown in FIG. 1, the semiconductor lightemitting element 2 is flip-chip mounted with the substrate 4 facing up,and therefore light emitting from the substrate 4 is extracted outsidethrough the interface between the substrate 4 and the fluorescentmaterial layer 16 and the interface between the fluorescent materiallayer 16 and the scattering layer 21. Light entering from a mediumhaving a higher refractive index into a medium having a lower refractiveindex undergoes total reflection at an interface when the incident angleis larger than the critical angle that is determined by the differencein refractive index. Therefore, in order to efficiently extract theprimary light emitted by the semiconductor light emitting element 2 tothe outside of the light emitting device 1, it is preferable that therefractive indices of the scattering layer 21 and the fluorescentmaterial layer 16 are as proximate to the refractive index of thesubstrate 4 as possible. Refractive index of a material used in thesubstrate 4 of the semiconductor light emitting element 2 is usuallyabout from 1.7 to 2.5 (about 1.76 in the case of sapphire). Refractiveindex of the scattering layer 21 can be increased from the level of 1.41to 1.53 of the transparent medium 18 to a level proximate to therefractive index of the substrate 4, by dispersing the particles 20having a high refractive index in the transparent medium 18. Differencein effective refractive index between the scattering layer 21 and thesubstrate 4 is preferably 0.2 or less, and more preferably 0.05 or less.When the scattering layer 21 and the substrate 4 have refractive indicesproximate to each other, difference in effective refractive index ateach interface can be decreased so as to decrease the reflection loss,by setting the refractive index of the fluorescent material layer 16near to those of the members disposed on both sides thereof. Thisresults in an improvement in the light extracting efficiency of thelight emitting device 1. In the case of face-up mounting where thesubstrate 4 is disposed at the bottom, too, it is preferable that therefractive indices are set similarly. When the substrate of thesemiconductor light emitting element is removed to have only thesemiconductor layer remain, it is preferable that difference ineffective refractive index between the semiconductor layer thatconstitutes the semiconductor light emitting element and the scatteringlayer 21 is 0.2 or less, and more preferably 0.05 or less. In order toutilize the light emitting from the side face of the semiconductor lightemitting element, it is preferable to use the semiconductor lightemitting element that has a transparent substrate as in the lightemitting device shown in FIG. 1.

The respective members constituting the light emitting device 10 of thepresent embodiment will be described in detail below.

Scattering Layer 21

The scattering layer 21 is formed by dispersing the particles 20 in thetransparent medium 18. When the light emitting device emits light at alarge emitting angle as in the present embodiment, it is preferable tocoat the periphery of the semiconductor light emitting element 2 and thefluorescent material layer 16 with the scattering layer 21 over a widerange of angles. The scattering layer 21 is disposed in a region throughwhich light propagating within the light emitting angle of the lightemitting device 1 passes, by taking into consideration the light pathfollowed by the primary light emitted by the semiconductor lightemitting element 2 that is not subjected to scattering. This region ispreferably coated entirely by the scattering layer 21. In the presentembodiment, the scattering layer 21 is formed in a semi-spherical shapeso as to entirely coat the semiconductor light emitting element 2 andthe fluorescent material layer 16. When the scattering layer 21 has asemi-spherical shape, reflection loss at the interface between thescattering layer 21 and the outside (air) decreases, which ispreferable. The scattering layer 21 may be disposed anywhere as long asit can scatter the primary light emitted by the semiconductor lightemitting element 2. Provided that, in order to improve the emissionintensity distribution of the semiconductor light emitting element 2, itis preferable that the semiconductor light emitting element 2 havingsquare shape is positioned at the center of the scattering layer 21 inplan view. It is also preferable that the scattering layer 21 coats thetop surface and the side faces of the semiconductor light emittingelement 2 in sectional view. The scattering layer 21 is disposed on thelight emitting side of the fluorescent material layer 16. The lightemitting side of the fluorescent material layer 16 refers to the side ofthe principal surface opposite to the surface that receives the primarylight emitted by the semiconductor light emitting element 1, among thetwo principal surfaces of the fluorescent material layer 16. Bydisposing the scattering layer 21 on the light emitting side of thefluorescent material layer 16, it is made possible to improve theemission intensity distribution of the primary light that has passed thefluorescent material layer 16 and it becomes easier to suppress thecolor unevenness in the final output. On the other hand, since thescattering layer 21 does not strongly scatter the secondary light of thefluorescent material layer 16, the light that returns due to excessivescattering of the secondary light can also be suppressed fromincreasing. It is preferable to coat the entire principal surface on thelight emitting side of the fluorescent material layer 16 with thescattering layer 21.

(a) Particles 20

The particles 20 can scatter the light if it is formed from a materialhaving a refractive index different from that of the transparent medium18 wherein the particles are dispersed, but the material preferably hasa refractive index higher than that of the transparent medium 18. Thisenables it to further increase the light extracting efficiency of thelight emitting device 1 and improve reliability. That is, it is madepossible to increase the effective refractive index of the scatteringlayer 21 and improve the light extracting efficiency when the particles20 have a high refractive index. This widens a selection range of thematerial of the transparent medium 18, and makes it possible to use amaterial having higher durability for the transparent medium 18.

It is also possible to use, as particles 20, various materials such asinorganic materials, organic materials and composite materials. It ispreferred that particles 20 are formed from the inorganic material sincedurability, heat conductivity and refractive index of the particles 20itself are enhanced. If heat conductivity of the particles 20 becomeshigher, efficiency of heat dissipation through the scattering layer 21is also improved.

It is preferred to use, as the material of the particles 20, inorganicmaterials having a high refractive index (particularly, oxide, nitride,sulfide). Examples thereof include titanium oxide, niobium oxide,aluminum oxide, yttrium oxide, zirconium oxide, diamond, tantalum oxide,cerium oxide, yttrium aluminum garnet (YAG), yttrium vanadate (YVO₄),indium oxide, zinc sulfide and silicon nitride. Of these materials,zirconium oxide and niobium oxide are preferably used. It is preferredto use, as the material of the articles 20, materials different form thefluorescent material.

It is desirable that refractive index of the particles 20 is 1.7 ormore, and more preferably 2.0 or more. This is because, the more therefractive index of the particles 20 than that of the transparent medium18, the more strongly the primary light is scattered by the particles 20and the effective refractive index of the scattering layer 21 increases.A level of refractive index of the particles 20 used commonly would notbe regarded as being too large. As refractive index of the particles 20increases, difference thereof from that of the transparent medium 18increases and strength of scattering increases. When the strength ofscattering is too large, it can be controlled within a proper range byadjusting the number of particles 20. Such a level of refractive indexis desirable particularly when peak wavelength of the primary lightemitted by the semiconductor light emitting element is from 420 to 500nm.

It is necessary that mean particle size D of the particles 20 satisfyInequality 1. However, it is preferable that mean particle size D islarger within the range of Inequality 1, in order to increase thestrength of scattering the primary light emitted by the semiconductorlight emitting element. The mean particle size D is preferably 25 nm ormore, and more preferably 30 nm or more. When the particle size of theparticle 20 is not spherical, the particle size is determined in termsof a maximum size of the particle 20.

The amount (% by weight) of the particles 20 to be dispersed in thetransparent medium 18 is 10% or more, preferably 50% or more. This isbecause the strength of scattering light by the particles 20 dependsalso on the number of particles 20 as implied by Equation 2. Heatconductivity of the scattering layer 21 is improved as the amount of theparticles 20 to be dispersed increases. When the amount of particles 20to be dispersed is too large, light returning from the scattering layer21 to the semiconductor light emitting element 2 and the fluorescentmaterial layer 16 increases, and the light extracting efficiencydecreases. Thus amount of the particles 20 to be dispersed is preferably80% or less, and more preferably 70% or less.

The object of the scattering layer 21 of this embodiment is to scatterthe primary light of the semiconductor light emitting element 2 and letit emit to the outside, and therefore it is important to prevent theprimary light of the semiconductor light emitting element 2 that returnsback from increasing and causing the utilization rate of thesemiconductor light emitting element 2 to decrease. For this purpose, itis preferable to control the number, particle size, refractive index orthe like of the particles 20 dispersed in the scattering layer 21 sothat 80% or more, more preferably 90% or more of the primary lightemitted by the semiconductor light emitting element 2 can emit to theoutside of the light emitting device 1.

There is no restriction on the distribution of the particles 20 in thescattering layer 21. It is advantageous to have such a distribution asthe density decreases from the semiconductor light emitting element 2toward the side where the light is extracted (air side), as thisprovides a high effective refractive index on the side of thesemiconductor light emitting element 2 (refractive index of 1.76) and alower effective refractive index on the side that is in contact with theoutside air (refractive index 1).

(b) Transparent Medium 18

It is preferable that the transparent medium 18 is colorless and hashigh transmittance, homogeneity, high isotropy and high durability.However, there is a limitation on the material that satisfies theseoptical properties, and the refractive index is in a range from about1.4 to 1.54. These values are lower than those of sapphire and groupIII-V semiconductors that are commonly used in semiconductor lightemitting elements for emitting visible light of short wavelengths.Therefore, an interface with a large difference in a refractive indexexists in the path of light propagating from the semiconductor lightemitting element through the transparent resin to the outside of thelight emitting device, and extraction of light is impeded by totalreflection taking place at the interface. This also decreases the lightextracting efficiency of the light emitting device. In the presentembodiment, it is possible to increase light extracting efficiency byincreasing a refractive index of the transparent medium 18, since theparticles 20 having high refractive index are dispersed in thetransparent medium 18.

It is possible to use, as the material of the transparent medium 18,organic materials and inorganic materials that have transparency tolight of the semiconductor light emitting element 2. As the organicmaterial, resins having transparency can be used. It is preferred to usea silicone resin composition and a modified silicone resin composition.It is also possible to use insulating resin compositions havingtransparency, such as an epoxy resin composition, a modified epoxy resincomposition and an acrylic resin composition. It is also possible toutilize resins having excellent weatherability, such as a hybrid resincontaining at least one kind of these resins. It is possible to use, asthe inorganic material, amorphous materials such as glass, inorganiccrystals and ceramics. It is preferred to use, as the material of thetransparent medium 18, a silicone resin composition and a modifiedsilicone resin composition. It is preferred to use insulating resincompositions having transparency, such as an epoxy resin composition, amodified epoxy resin composition and an acrylic resin composition sincethe insulating resin compositions are colorless and transparent evenafter curing and have excellent properties of an optical material, suchas homogeneity and high isotropy, and also inexpensive and are easilymolded. Although these materials have a low refractive index of about1.4 to 1.54, it is possible to increase an effective refractive index bydispersing particles having a high refractive index.

It is possible to enhance reliability of the light emitting device byusing, as the material of the transparent medium 18, a silicone resincomposition having no phenyl group introduced thereinto, a modifiedsilicone resin composition having no phenyl group, for example, amethyl-based silicone resin composition or a methyl-based modifiedsilicone resin composition by the following reason. When the phenylgroup is introduced, the refractive index of the transparent medium 18increases, while light resistance and heat resistance of the transparentmedium 18 deteriorate. It is possible to increase the effectiverefractive index of the scattering layer 21 by dispersing particles 20having a high refractive index without introducing the phenyl group intothe transparent medium 18.

It is possible to increase the refractive index of the transparentmedium 18 by using, as the material of the transparent medium 18, asilicone resin composition having a phenyl group introduced thereinto ora modified silicone resin composition having a phenyl group introducedthereinto. It is possible to further increase the effective refractiveindex of the scattering layer 21 and to improve light extractingefficiency of the light emitting device 1 by dispersing particles 20having a high refractive index.

The effective refractive index of the scattering layer 21 can beestimated from the relative dielectric constant of the transparentmedium 18 and the relative dielectric constant of the particles 20, onthe basis of Maxwell-Garnet theory. It is also important that thetransparent medium 18 and the particles 20 are transparent to theprimary light of the semiconductor light emitting element 2. However,these members may be formed from materials that absorb ultraviolet raysbeyond the visible region, since the primary light of the semiconductorlight emitting element 2 is visible light.

Fluorescent Later 16

When the light emitting device has a large emission angle as in thepresent embodiment, it is preferable to coat the periphery of thesemiconductor light emitting element 2 with the fluorescent materiallayer 16 over a wide range of angles. It is also preferable to coat theentire region through which light propagating within the light emittingangle of the periphery the light emitting device 1 passes, by takinginto consideration the light path followed by the primary light emittedby the semiconductor light emitting element 2 in case it is notscattered. The fluorescent material layer 16 may be provided in variousforms. For example, the fluorescent material layer may be formed byhaving fluorescent material particles of inorganic material included ina fluorescent material holding member formed from a transparent resin.The fluorescent material layer 16 can be formed around the semiconductorlight emitting element 2 by various processes such as potting, screenprinting or the like. There is no limitation on the fluorescent materiallayer 16 as long as it is capable of absorbing a part of light emittedby the semiconductor light emitting element 2 and emitting light of alonger wavelength. The fluorescent material layer 16 may be formed byhaving fluorescent material particles 14 contained in the fluorescentmaterial holding member 12 such as glass or resin as in the presentembodiment, or formed from a fluorescent material in crystal or anamorphous material.

It is preferable to use, as the fluorescent material particles 14, afluorescent material that is excited by blue light and emits yellowlight of broad spectrum, since white light having high color renderingperformance is obtained. It is particularly preferable to usefluorescent material particles 14 formed from an inorganic material,because an inorganic material can endure intense heat or light producedby the semiconductor light emitting element 2. Wavelength of peakemission of the fluorescent material preferably is from 500 to 600 nm,more preferably from 520 to 560 nm. The fluorescent material particles14 may be formed from, for example, a fluorescent material having garnetstructure activated with cerium (particularly a fluorescent materialhaving garnet structure that is activated with cerium and containsaluminum). A fluorescent material activated with cerium has broademission spectrum in yellow region, and is therefore capable ofproducing white light of high color rendering performance when combinedwith blue light emission. A fluorescent material having garnetstructure, particularly garnet structure containing aluminum, is durableagainst heat, light and moisture and can maintain the emission of yellowlight with high brightness over a long period of time. As the wavelengthconverting material, for example, it is preferable to use a YAGfluorescent material (usually abbreviated to YAG) having composition of(Re_(1-x)Sm_(x))₃(Al_(1-y)Ga_(y))₅O₁₂:Ce (0≦x<1, 0≦y≦1, where Re is atleast one element selected from the group consisting of Y, Gd, La, Luand Tb). Color rendering performance may also be adjusted by using afluorescent material such as Lu₃Al₅O₁₂:Ce, BaMgAl₁₀O₁₇:Eu,BaMgAl₁₀O₁₇:Eu, Mn, (Zn, Cd)Zn:Cu, (Sr, Ca)₁₀(PO₄)₆Cl₂:Eu, Mn, (Sr,Ca)₂Si₅N₈:Eu, CaAlSiB_(x)N_(3-x):Eu, and CaAlSiN₃:Eu, other than theyellow fluorescent material.

It is possible to use, as the material of the fluorescent materialholding member 12 containing fluorescent material particles 14, organicmaterials and inorganic material that have transparency to light of thesemiconductor light emitting element 2. As the organic material, a resinhaving a transparency is preferred. For example, it is preferred to usea silicone resin composition and a modified silicone resin composition.However, it is possible to use insulating resin compositions havingtransparency, such as an epoxy resin composition, a modified epoxy resincomposition and an acrylic resin composition. It is also possible toutilize resins having excellent weatherability, such as a hybrid resincontaining at least one kind of these resins. It is also possible touse, as the inorganic material, amorphous materials such as glass,inorganic crystals and ceramics. As described above, when the crystal ofthe fluorescent material or amorphous material itself is used as thematerial of the fluorescent material layer 16, a fluorescent materialholding member 12 becomes unnecessary.

Semiconductor Light Emitting Element 2

The semiconductor light emitting element 2 may be one that is providedwith a light emitting layer formed from a semiconductor. Particularly alight emitting element having a light emitting layer formed from anitride semiconductor, and a light emitting layer formed from a nitridegallium compound semiconductor (particularly InGaN) above all, can emitintense light in blue region, and therefore can be advantageouslycombined with the fluorescent material layer 16. While it is sufficientthat the light emitting layer 24 of the semiconductor light emittingelement 2 emits light with peak wavelength in the visible region, it isdesirable that the peak wavelength is from 420 nm to 500 nm, morepreferably from 445 nm to 465 nm. When the semiconductor light emittingelement 2 emits blue light in this wavelength region, it can emit lightof desired color, particularly white light, by combining it with variouskinds of fluorescent material layer 16. The semiconductor light emittingelement 2 may also have a light emitting layer formed from a ZnSe-based,InGeAs-based or AlInGaP-based semiconductor. The semiconductor lightemitting element 2 is preferably a light emitting diode of surfaceemission type, where light is extracted from the bottom surface of thesubstrate 4 or the top surface of the semiconductor layer 6.

In the present embodiment, the light emitting element 2 is flip-chipmounted on the mounting substrate 10 with the substrate 4 facing up, asshown in FIG. 1. The mounting substrate 10 has electrodes (not shown)formed on the top surface thereof, which are connected to the p-sideelectrode 30 and the n-side electrode 32 of the semiconductor lightemitting element 2 via the solder bumps 8.

The semiconductor light emitting element 2 that can be used in thepresent invention is not limited to one that has the structure shown inFIG. 2. For example, insulating, semi-insulating or reverse conductivitytype structure may be provided in a part of the layer of eachconductivity type. The substrate 4 may also be electrically conductive,in which case the n-side electrode 32 may be formed on the back surfaceof the substrate 4. The substrate 4 may be either the substrate used togrow the semiconductor layer 6 or may be attached after growing thesemiconductor layer 6. The substrate 4 may be removed. For example,after the light emitting element 2 including the substrate 4 has beenflip-chip mounted, the substrate 4 may be peeled.

Mounting Substrate 10

The mounting substrate 10 may have wiring formed on the surface thereoffor electrical connection with the semiconductor light emitting element2. In the present embodiment, the mounting substrate 10 is made byforming the wiring on a flat insulating member. It is possible to use,as the insulating member, ceramics such as aluminum nitride or alumina,or glass. The mounting substrate 10 may also be used by forming aninsulating thin film such as aluminum nitride on the surface ofsemi-metal such as Si or a metal. The mounting substrate 10 having sucha constitution is preferable because of high heat dissipation. Thewiring may be formed by subjecting a metal layer to patterning using anion milling method or an etching method. For example, the wiring patternmay be formed from a thin film of platinum or the like on the surface ofaluminum nitride. In addition, a protective film may also be formed froma thin film such as SiO₂ for the purpose of protecting the wiring.

Second Embodiment

FIG. 4 is a sectional view showing a light emitting device according tothe second embodiment of the present invention. The light emittingdevice 1 shown in FIG. 4 comprises two layers; a first fluorescentmaterial layer 16 a that emits red light and a second fluorescentmaterial layer 16 b that emits yellow light, disposed in this order fromthe inside. Fluorescent material layer 16 a is formed by dispersingfluorescent material particles 14 a in a fluorescent material holdingmember 12 a, and fluorescent material layer 16 b is formed by dispersingfluorescent material particles 14 b in a fluorescent material holdingmember 12 b. This constitution enables it to make the light emittingdevice that emits light with higher red component and higher mean colorrendering index Ra than in the case of forming only the fluorescentmaterial layer 16 that emits yellow light. The light emitting devicehaving higher mean color rendering index Ra is suited for illuminationapplications. Increasing the red component also enables it to make thelight emitting device that emits light of warm white. This embodiment issimilar to the first embodiment in other respects.

The first fluorescent material layer 16 a preferably containsfluorescent material particles 14 a that emit light in yellow to redcolor range. Fluorescent materials that transform visible light intolight in yellow to red color range include a nitride fluorescentmaterial, an oxynitride fluorescent material and a silicate fluorescentmaterial.

Examples of nitride fluorescent material and oxynitride fluorescentmaterial include Sr—Ca—Si—N:Eu, Ca—Si—N:Eu, Sr—Si—N:Eu, Sr—Ca—Si—O—N:Eu,Ca—Si—O—N:Eu, Sr—Si—O—N:Eu, etc. Of the nitride fluorescent material andthe oxynitride fluorescent material, an alkaline earth-silicon nitridefluorescent material is preferred and is represented by the followingformula: LSi₂O₂N₂:Eu, L_(x)Si_(y)N_((2/3x+4/3y)):Eu,L_(x)Si_(y)O_(z)N_((2/3x+4/3y-2/3z)):Eu (L represents one of Sr, Ca, orSr and Ca).

As the silicate fluorescent material, L₂SiO₄:Eu (L represents analkaline earth metal), (Sr_(x)Mae_(1-x))₂SiO₄:Eu (Mae represents analkaline earth metal such as Ca or Ba) is preferably used.

As the second fluorescent material layer 16 b, fluorescent materialparticles 14 b that are excited by blue light and emit yellow light inbroad spectrum are used. As the fluorescent material particles 14 b, amaterial similar to those described in the first embodiment may be used.

In the present embodiment, while a case where the first fluorescentmaterial layer 16 a that emits red light and the second fluorescentmaterial layer 16 b that emits yellow light are provided has beendescribed, the present invention is not limited to thereto. Anyfluorescent materials may be used as long as they are two fluorescentmaterials with different light emission wavelengths. Use of two kinds offluorescent material that emit light of different wavelengths improvescolor rendering performance of the light emitting device. In the presentembodiment, a case where two kinds of fluorescent material particles 14a, 14 b are held on the different fluorescent material layers 16 a, 16b, although two kinds of fluorescent material particles may also becontained in single fluorescent material layer 16. Such a constitutionmay also be implemented as the primary light emitted by thesemiconductor light emitting element 2 excites a first fluorescentmaterial which emits the secondary light for exciting a different kindof fluorescent material to emit light. Use of two kinds of fluorescentmaterial having different chromaticity values makes it possible to emitlight with any chromaticity within a region defined by connecting thepoints of chromaticity of the two kinds of fluorescent material and ofthe semiconductor light emitting element on the chromaticity diagram.

Third Embodiment

FIG. 5 is a sectional view showing a light emitting device according tothe third embodiment of the present invention. The light emitting device1 shown in FIG. 5 has particles 20 formed from an inorganic materialhaving high refractive index dispersed also in the fluorescent materiallayer 16. This embodiment is similar to the first embodiment except forthis respect.

Refractive index of the fluorescent material layer 16 can be increasedby dispersing the particles 20 having a higher refractive index thanthat of the fluorescent material holding member 12. This increases thecritical angle at which total reflection occurs in the interface betweenthe substrate 4 and the fluorescent material layer 16 and the interfacebetween the fluorescent material layer 16 and the scattering layer 21,thereby decreasing the reflection loss at the interfaces. When theparticles 20 formed from an inorganic material are dispersed also in thefluorescent material layer 16, the heat conductivity of the fluorescentmaterial layer 16 can be increased and also reliability of the lightemitting device is improved. Furthermore, when the particles 20 are alsodispersed in the fluorescent material layer 16, emission intensitydistribution of the primary light of the semiconductor light emittingelement 2 is improved and the proportion of the primary light enteringthe fluorescent material particles 14 in the fluorescent material layer16 also increases. Moreover, mean particle size D of the particles 20dispersed also in the fluorescent material layer 16 satisfies Inequality1 and the particles do not strongly scatter the secondary light oflonger wavelength emitted by the fluorescent material particles 14, andtherefore light extracting efficiency can be suppressed from decreasingdue to the increase in excessive return light.

In order to disperse the particles 20 in the fluorescent material layer16 in the present embodiment, the particles 20 may be dispersed alongwith the fluorescent material particles 14 when forming the fluorescentmaterial layer 16 by potting or printing. When the fluorescent materiallayer 16 is formed by electrophoresis, a resin containing the particles20 may be impregnated after forming the fluorescent material layer 16.For example, dipping in the uncured solution of the transparent medium18 containing the particles 20 may be performed after forming thefluorescent material layer 16. The particles 20 dispersed in thefluorescent material layer 16 may be the same as or different from theparticles 20 dispersed in the scattering layer 21, as long as thematerial satisfies Inequality 1. Provided that it is advantageous to usethe same material as the particles 20 dispersed in the scattering layer21, since it simplifies the process of manufacturing the light emittingdevice 1. It is preferable that concentration of the particles 20dispersed in the fluorescent material layer 16 is higher than theconcentration of the particles 20 dispersed in the scattering layer 21.Refractive index can be increased by including more particles 20 havinghigh refractive index. Thus it is made possible to extract the primarylight and the secondary light efficiently to the outside of the lightemitting device 1 by making the refractive index of the scattering layer21, that is disposed on the light extracting side of the light emittingdevice 1, lower than the refractive index of the fluorescent materiallayer 16, since the refractive index of the light emitting device 1becomes nearer to the refractive index of air with the position from thesemiconductor light emitting element 2 toward the light extracting side.

Fourth Embodiment

FIG. 6 is a schematic sectional view showing a light emitting deviceaccording to the fourth embodiment of the present invention. In thepresent embodiment, the semiconductor light emitting element 2 ismounted with the semiconductor layer 6 facing up, and the particles 20are dispersed in a die-bonding material 38 that bonds the semiconductorlight emitting element 2 onto the mounting substrate 10. This embodimentis similar to the third embodiment in other respects.

The substrate 4 of the semiconductor light emitting element 2 isfastened onto the surface of the mounting substrate 10 by means of thedie-bonding material 38, as shown in FIG. 6. The die-bonding material 38is applied to nearly the entire bottom surface of the semiconductorlight emitting element 2. A p-side electrode and an n-side electrode(not shown) formed on the semiconductor layer 6 of the semiconductorlight emitting element 2 are connected to the electrodes (not shown)formed on the mounting substrate 10 by wires 36. The die-bondingmaterial 38 that fastens the semiconductor light emitting element 2 ontothe mounting substrate 10 contains the particles 20 formed from aninorganic material having a higher refractive index than that of theresin that constitutes the die-bonding material 38 dispersed therein.The periphery of the die-bonding material 38 is coated by thefluorescent material layer 16 and the scattering layer 21.

When the particles 20, formed from an inorganic material having a higherrefractive index than that of the resin that constitutes the die-bondingmaterial 38, are dispersed in the die-bonding material 38 that fastensthe semiconductor light emitting element 2 onto the mounting substrate10 as in the present embodiment, refractive index of the die-bondingmaterial 38 becomes proximate to the refractive index of the substrate 4of the semiconductor light emitting element 2. This decreases the lightreflected on the interface between the substrate 4 and the die-bondingmaterial 38, thereby decreasing the light returning into thesemiconductor light emitting element 2 and improving the lightextracting efficiency. Also as the primary light of the semiconductorlight emitting element 2 is scattered in die-bonding material 38, thelight arriving at the metal electrode provided on the surface of themounting substrate 10 decreases and absorption loss therein decreases.Also because the refractive index of the die-bonding material 38 becomesproximate to the refractive index of the substrate 4 and the primarylight is scattered by the particles 20 in the die-bonding material 38,such an effect is achieved that would be obtained by providing surfaceroughness on the back surface of the substrate 4 for the prevention ofmultiple reflections. As a result, absorption loss due to multiplereflections in the semiconductor light emitting element 2 can bedecreased and light extracting efficiency of the light emitting device 1can be improved. Also as the particles 20 formed from an inorganicmaterial are dispersed in the die-bonding material 38, heat conductivityof the die-bonding material 38 increases and therefore heat dissipationof the light emitting device 1 is improved, resulting in improvedreliability. Thickness of the die-bonding material 38 may be, forexample, 2 μm or less which is far thinner than the scattering layer 21and the fluorescent material layer 16. As a result, density of theparticles 20 in the die-bonding material 38 can be made higher thanthose in the scattering layer 21 and the fluorescent material layer 16.

When the semiconductor light emitting element 2 is mounted with thesemiconductor layer 6 facing up as in the present embodiment, differencein effective refractive index between the scattering layer 21 and thesubstrate 4 is preferably 0.2 or less, and more preferably 0.05 or less,similarly to the embodiments described above by the following reason.That is, when the substrate 4 is formed from a transparent material, thesubstrate 4 having a refractive index proximate to that of thesemiconductor layer 6 is selected. When the substrate 4 and thescattering layer 21 have refractive indices that are proximate to eachother, difference in effective refractive index at each interface can bedecreased so as to decrease the reflection loss, by setting therefractive index of the fluorescent material layer 16 disposed betweenthe substrate 4 and the scattering layer 21 near to those of thesemembers. This results in an improvement in light extracting efficiencyof the light emitting device 1. The difference in effective refractiveindex between the semiconductor layer 6 and the scattering layer 21 canbe set to 0.2 or less, and more preferably 0.05 or less, based on theeffective refractive index of the semiconductor layer 6.

The particles 20 dispersed in the die-bonding material 38 may be onethat satisfies Inequality 1, and may be the same as or different fromthe particles 20 dispersed in the scattering layer 21 and in thefluorescent material layer 16. While a case of face-up mounting of thesemiconductor light emitting element 2 is described in the presentembodiment, constitution of the present embodiment may also be appliedto the resin filled between the semiconductor light emitting element andthe mounting substrate in the case of face-down mounting.

Fifth Embodiment

FIG. 7 is a schematic sectional view showing a light emitting deviceaccording to the fifth embodiment of the present invention. In thepresent embodiment, the semiconductor light emitting element 2 isflip-chip mounted with the substrate 4 facing up, and the sheet-shapedfluorescent material layer 16 a is bonded onto the top surface of thesubstrate 4 via an adhesive layer 40. The sheet-shaped fluorescentmaterial layer 16 b is also bonded onto four side faces of thesemiconductor light emitting element 2 via the same adhesive layer 40.The sheet-shaped fluorescent material layers 16 a, 16 b have box shapethat opens at the bottom. This embodiment is similar to the firstembodiment in other respects. Also in the present embodiment, thefluorescent material layer 16 has one flat light emitting surface thatis nearly parallel to the principal surface of the semiconductor lightemitting element 2 on the light emitting side thereof, and four flatlight emitting surfaces that are nearly parallel to the side faces ofthe semiconductor light emitting element 2, similarly to the firstembodiment.

In the present embodiment, the particles 20 formed from an inorganicmaterial having a higher refractive index than that of the material ofthe adhesive layer 40 are dispersed in the adhesive layer 40 that bondsthe sheet-shaped fluorescent material layers 16 a, 16 b and thesemiconductor light emitting element 2. This constitution increases theeffective refractive index of the adhesive layer 40, and thereforedecreases the difference in effective refractive index in the interfacebetween the semiconductor light emitting element 2 and the adhesivelayer 40 and the interfaces between the adhesive layer 40 and thefluorescent material layers 16 a, 16 b, thereby increasing the criticalangle of total reflection and decreasing the return light due toreflection. Also because blue light that has been scattered by theadhesive layer 40 enters the fluorescent material layers 16 a, 16 b, thefluorescent material layers 16 a, 16 b are excited more uniformly andemission intensity distribution of the fluorescent material layers 16 a,16 b is improved. Also the primary light of the semiconductor lightemitting element 2 is scattered twice in the adhesive layer 40 and inthe scattering layer 21, and therefore emission intensity distributionof the semiconductor light emitting element 2 becomes more uniform. Theefficiency of heat dissipation is also improved in the path from thesemiconductor light emitting element 2 to the adhesive layer 40 and thefluorescent material layers 16 a, 16 b.

The sheet-shaped fluorescent material layers 16 a, 16 b used in thepresent embodiment are preferably formed from an inorganic material,particularly an inorganic material that has heat conductivity of 0.8W/mK or more, preferably 1.0 W/mK or more, and more preferably 4.0 W/mKor more. Specifically, the fluorescent material layers 16 a, 16 b may beformed from a crystal or an amorphous material of the inorganicfluorescent material, or the fluorescent material layers 16 a, 16 b maybe formed by including inorganic fluorescent material particles in atransparent member formed from an inorganic material such as glass oralumina. An example of the fluorescent material layer s 16 a, 16 bformed from a crystal or amorphous material of the inorganic fluorescentmaterial is a sheet-shaped body formed from crystal of fluorescentmaterial having garnet structure activated with cerium. When theinorganic fluorescent material particles are contained in a transparentmember formed from an inorganic material, an amorphous material such asglass, inorganic crystal or ceramics may be used as the transparentmember of the inorganic material. When the fluorescent material layers16 a, 16 b are formed from such a material, durability of thefluorescent material layers 16 a, 16 b itself is improved and heatdissipation from the fluorescent material layers 16 a, 16 b toward themounting substrate 10 is improved, so that the light emitting device 1of higher reliability can be realized. In order that the fluorescentmaterial layers 16 a, 16 b have “sheet shape”, the overall shape may besheet shape that may have recess or hole in the surface thereof, and apattern for achieving some optical effect may be formed on the surface.When fluorescent material layers 16 a, 16 b have “sheet shape”, it isnot limited to square and may have various shapes such as round or ovalin a plan view. The sheet-shaped fluorescent material layer 16 ispreferably disposed in parallel to the principal surface of thesubstrate of the semiconductor light emitting element 2. If thefluorescent material layer 16 has sheet shape, the fluorescent materiallayer 16 can be formed simply by bonding the sheet-shaped body onto thesemiconductor light emitting element 2. When the fluorescent materiallayer 16 is formed in sheet shape, it becomes easier to assemble thelight emitting device since the fluorescent material layer 16 can beprepared in a sheet member having a little larger size which may be cutinto a desired size and used to manufacture the light emitting device 1.

The fluorescent material layer 16 b formed on the side face of thesemiconductor light emitting element 2 may be formed by printing or thelike, not by bonding a sheet-shaped member. The particles 20 dispersedin the bonding layer 40 may be the same as or different from theparticles 20 dispersed in the scattering layer 21, as long as Inequality1 is satisfied.

Sixth Embodiment

FIG. 8 is a schematic sectional view showing a light emitting device 1according to the sixth embodiment of the present invention. In thepresent embodiment, a reflecting member 45 is formed to reflect thesecondary light of the fluorescent material layer 16 a together with theprimary light, while scattering the light, instead of the fluorescentmaterial layer 16 b formed on the side face of the semiconductor lightemitting element 2. The scattering layer 21 coats only the top surfaceof the fluorescent material layer 16 a that is disposed upward thesemiconductor light emitting element 2, and is not formed on the side ofthe semiconductor light emitting element 2. Thus, in the presentembodiment, the secondary light from the fluorescent material layer 16 ais extracted through one flat light extracting surface that is nearlyparallel to the principal surface of the semiconductor light emittingelement 2. In this case, too, for a reason similar to that described inthe first embodiment, the secondary light has higher intensity in theoblique direction than in the direction perpendicular to the principalsurface of the semiconductor light emitting element 2, so that emissionintensity distribution of the secondary light of the fluorescentmaterial layer 16 shows a trend opposite to the emission intensitydistribution of the primary light of the semiconductor light emittingelement 2. Therefore, color unevenness is likely to occur according tothe reason similar to the first to fifth embodiments. This embodiment issimilar to the fifth embodiment in other respects.

When the fluorescent material layer 16 a has sheet shape as in thepresent embodiment, emission intensity tends to increase on the endfaces of the sheet, in consideration of the distribution within plane.Accordingly, when the fluorescent material layer 16 a has sheet shapeand is fastened onto the top surface of the semiconductor light emittingelement as in the present embodiment, it is preferable to form thescattering layer 21 on the top surface of the fluorescent material layer16 a, and provide the reflecting member 45 on the side face of thefluorescent material layer 16 a so as to reflect the secondary light ofthe fluorescent material layer 16 a by the reflecting member 45 whilescattering the light. It is preferred that the reflecting member 45 isalso provided on the side face of the semiconductor light emittingelement 2, as shown in FIG. 8.

In the reflecting member 45, scattering particles 44 having relativelylarge particle size are dispersed in a resin 42. Mean particle size ofthe scattering particles 44 is set to such a level that causes Miescattering or diffraction scattering of blue light emitted by thesemiconductor light emitting element 2, so that both the blue lightemitted by the semiconductor light emitting element 2 and light oflonger wavelength emitted by the fluorescent material layer 16 a arescattered. Specifically, mean particle size D′ of the scatteringparticles 44 is set to 70 nm or more, preferably 200 nm or more. Whenthe mean particle size of the scattering particles 44 is 70 nm or more,visible light can be subjected to Mie scattering or diffractionscattering, if the transparent medium 42 has refractive index of 1.4 orhigher. While the scattering layer 21 selectively scatter the primarylight of the semiconductor light emitting element, the reflecting member45 can reflect both the primary light of the semiconductor lightemitting element 2 and the secondary light of the fluorescent materiallayer 16 a, on the reflecting member 45 while scattering the light. Thescattering particles 44 of the reflecting member 45 may be formed from amaterial similar to that of the particles 20 of the scattering layer 21with a larger mean particle size, so as to function in the Miescattering or diffraction scattering regime. The resin 42 of thereflecting member 45 may also be a material similar to that of thetransparent medium 18 of the scattering layer 21.

The reflecting member 45 is formed on the side of the semiconductorlight emitting element 2. Therefore the reflecting member 45 is capableof reflecting the primary light emitted from the side face of thesemiconductor light emitting element 2 and the secondary light emittedfrom the side face and bottom surface of the fluorescent material layer16 a while scattering the light, so that emission with less colorunevenness can be obtained. On the other hand, blue light emitted fromthe top surface of the semiconductor light emitting element 2 passesthrough the fluorescent material layer 16 a and then through thescattering layer 21 that selectively scatters the primary light of thesemiconductor light emitting element 2, and emits therefrom. Accordingto the present embodiment, the light emitting device having less colorunevenness can be made in a simpler constitution than that of the fifthembodiment. It is preferable that the reflecting member 45 coats notonly the side faces of the semiconductor light emitting element 2 butalso the side face of the plate-shaped fluorescent material layer 16 a.This is because light emitted from the side face of the sheet-shapedfluorescent material layer 16 a includes light of longer wavelengths inhigher proportion to the blue light of the semiconductor light emittingelement 2, and is likely to cause color unevenness. In the constitutionshown in FIG. 8, top surface of the sheet-shaped fluorescent materiallayer 16 a is coated by the scattering layer 21, while the side facesand a part of the bottom surface are coated by the reflecting member 45.It is preferable that the reflecting member 45 coats the periphery ofthe semiconductor light emitting element 2 and the fluorescent materiallayer 16 in an isotropic manner in plan view.

In the present embodiment, since the side faces of the semiconductorlight emitting element 2 are exposed without being coated by thefluorescent material layer 16, the proportion of the primary light ofthe semiconductor light emitting element 2 that is absorbed by thefluorescent material layer 16 is reduced and light extracting efficiencycan be improved.

Seventh Embodiment

FIG. 9 is a schematic sectional view showing a light emitting device 1according to the seventh embodiment of the present invention. In thepresent embodiment, a concave mounting substrate 10 having inversetrapezoidal shape in the cross section with the surface thereof formedas reflecting mirror is used. After mounting the semiconductor lightemitting element 2 on the mounting substrate 10, the transparent medium18 containing both the fluorescent material particles 14 and theparticles 20 mixed therein is potted and the fluorescent materialparticles 14 are allowed to precipitate. This embodiment is similar tothe first embodiment in other respects.

According to the present embodiment, the fluorescent material layer 16and the scattering layer 21 can be formed simultaneously with a simplemanufacturing method. In the present embodiment, the fluorescentmaterial layer 16 is constituted from fluorescent material particles 14that precipitate so as to coat the top surface and the side faces of thesemiconductor light emitting element 2. To manufacture the lightemitting device 1 by the method of the present embodiment, a materialhaving sufficiently low viscosity before curing may be used for thetransparent medium 18, so that the fluorescent material particles 14precipitate when potted on the mounting substrate 10 whereon thesemiconductor light emitting element 2 has been mounted. The particles20 have smaller particle size and therefore do not precipitate. Thusaccording to the present embodiment, the fluorescent material layer 16and the scattering layer 21 can be formed simultaneously in a singlemanufacturing process. When the fluorescent material layer 16 is formedby causing the fluorescent material particles 14 to precipitate,absorption loss of the primary light and the secondary light in thefluorescent material layer 16 can be reduced, favorably. The fluorescentmaterial particles 14 having spherical shape precipitate more easily ascompared with the uneven shape.

The embodiments described above are mere examples, and the presentinvention is not restricted to these embodiments. For example, thefluorescent material layer 16 explained in the fifth embodiment may beformed in the light emitting device of the first through fourthembodiments, and the fluorescent material layer 16 explained in thefirst through sixth embodiments may be mounted on the mounting substrate10 of the seventh embodiment. Components of the present invention arenot limited to the constitution based on the members of the embodimentsdescribed above, and a plurality of components of the present inventionmay be constituted from a single member, or one component may beconstituted from a plurality of members.

DESCRIPTION OF REFERENCE NUMERALS

-   1: Light emitting device-   2: Semiconductor light emitting element-   4: Substrate-   6: Semiconductor layer-   8: Solder bump-   10: Mounting substrate-   12: Fluorescent material holding member-   14: Fluorescent material particles-   16: Fluorescent material layer-   18: Transparent medium-   20: Particles-   21: Scattering layer-   22: n-side nitride semiconductor layer-   24: Active layer-   26: p-side nitride semiconductor layer-   28: p-side electrode-   30: p-side pad electrode-   32: n-side electrode-   34: Insulating protective film-   36: Wire-   38: Die-bonding material-   40: Adhesive layer-   42: Resin-   44 Scattering particles-   45 Reflecting member

What is claimed is:
 1. A method of manufacturing a light emitting devicecomprising: disposing a semiconductor light emitting element including asemiconductor layer that emits a primary light on a mounting substrate;covering the semiconductor light emitting element with a transparentmedium containing fluorescent material particles that absorb a part ofthe primary light and emits a secondary light having a wavelength longerthan that of the primary light and scattering particles having a meanparticle size D that satisfies Inequality 1:20 nm<D≦0.4×λ/π  [Inequality 1] where λ is the wavelength of the primarylight propagating in the transparent medium; and precipitating thefluorescent material particles so as to form a fluorescent layer wherethe fluorescent material particles are mainly dispersed in thetransparent medium and a scattering layer where the scattering particlesare mainly dispersed in the transparent medium.
 2. The method ofmanufacturing a light emitting device according to claim 1, wherein themount substrate forms a recess having inverse trapezoidal shape in thecross section.
 3. The method of manufacturing a light emitting deviceaccording to claim 1, wherein the scattering particles are formed froman inorganic material having a higher refractive index than that of thetransparent medium.
 4. The method of manufacturing a light emittingdevice according to claim 1, wherein the semiconductor light emittingelement includes a substrate, and a difference in an effectiverefractive index between the scattering layer and the substrate of thesemiconductor light emitting element is 0.2 or less.
 5. The method ofmanufacturing a light emitting device according to claim 1, wherein thescattering particles contain an oxide, sulfide or nitride having ahigher refractive index than that of the transparent medium.
 6. Themethod of manufacturing a light emitting device according to claim 1,wherein the scattering particles comprise at least one kind selectedfrom the group consisting of titanium oxide, niobium oxide, aluminumoxide, yttrium oxide, zirconium oxide, diamond, tantalum oxide, ceriumoxide, yttrium aluminum garnet, yttrium vanadate, zinc sulfide andsilicon nitride.
 7. The method of manufacturing a light emitting deviceaccording to claim 1, wherein the transparent medium comprises at leastone kind selected from the group consisting of a silicone resin, amodified silicone resin, an epoxy resin, a modified epoxy resin, anacrylic resin and a hybrid resin containing these resins.
 8. The methodof manufacturing a light emitting device according to claim 1, wherein apeak wavelength of the primary light emitted by the semiconductor lightemitting element is from 420 to 500 nm.